Field of the Invention
The present invention relates to optical supercontinuum generation or broadening of bandwidth of an optical signal whereby wavelength of the signal is broadened between about 2 and 14 microns.
Description of the Prior Art
Nonlinear optical phenomena can be used to convert light from one wavelength to another, but it can also convert a narrow bandwidth wavelength source into a broadband source. The generation of a broadband source through a combination of nonlinear phenomena is typically called supercontinuum generation. In supercontinuum generation, pulses of femtoseconds (fs) to nanoseconds (ns) are spectrally broadened by various nonlinear processes, including self-phase modulation, stimulated Raman scattering and four wave mixing, dependent on the pump temporal properties and the dispersion slope of the fiber. While supercontinuum generation is possible by focusing a high intensity light into a nonlinear medium, much broader bandwidths and significantly lower thresholds are possible when the pump is coupled into an optical fiber where the guiding characteristics of the fiber allow the pump to interact with the nonlinearities of the fiber materials over long lengths.
Among the various available high power laser pumps, microchip laser sources are compact, capable of pulse widths below 5 ns (typical values range from 40 ps to 5 ns), and repetition rates from Hz to MHz. Modal profile for these lasers is very good, usually being below M2<1.5. Multiple material systems allow for the development of microchip laser architectures with emission wavelength spanning visible to mid-IR. Such lasers are usually based on rare-earth elements such as Nd, Yb, Er, Dy, Pr, Sm, Eu, Ho, and Tm but may also include Cr, Fe, and other transition metal ions. These active dopants are supported in a host that can be a crystalline material such as yttrium aluminum garnet (YAG), yttrium lithium fluoride (YLF), yttrium orthovanadate (YVO4), yttrium aluminum perovskite, potassium-gadolinium tungstate (KGW), yttrium scandium gallium garnet (YSGG), ZnSe, and others; ceramic materials such as lutetium oxide, spinel, yttrium oxide, and others; glass materials such as germanates, fluorides, ZBLAN, chalcogenides, tellurites, and others. Other examples include transition metal (TM2+, e.g., Cr2+ or Fe2+) doped binary (e.g. ZnSe, ZnS, CdSe, CdS, ZnTe) and ternary (e.g., CdMnTe, CdZnTe, ZnSSe) chalcogenide crystals and ceramics.
Microchip lasers are a unique class of laser systems with many properties distinct from those from other laser architectures such as laser diode or fiber lasers. Of particular importance to the present invention are pulsed microchip lasers, as the peak power of these systems can be very high, easily exceeding kW peak powers in a package whose volume can be on the order of cm3. The high peak power avoids the need for further amplification of these laser systems, the short length of the microchip laser provides a short-pulse without the need to resort to optical pulse modulation, external pulse shaping elements or nonlinearly induced pulse break-up such as modulation instability.
Compared to focusing into a nonlinear medium, optical fibers allow for long interaction lengths through optical waveguiding. An optical fiber comprises a core surrounded by one or more claddings. Light travels in the core and is confined by the index difference between the core and cladding. Microstructured fiber or photonic crystal fiber is a fiber whereby the cladding (or claddings) comprises a geometric arrangement of air holes in the cladding glass. Inhibit coupling fiber is a hollow core fiber whereby the density of light states is reduced but non-zero and the modal overlap between the air guided mode and the substrate mode (cladding mode) is minimal, allowing light to be guided in the hollow mode with low loss.
Supercontinuum generation has been demonstrated in silica fiber in the visible and near infrared. Unfortunately, transmission of the silica glass matrix limits the supercontinuum generation to less than about 2 μm. For supercontinuum generation in the infrared, alternate technologies and materials are needed.
Chalcogenide fiber is one technology capable of transmission well beyond 2 μm. Chalcogenide fibers are fibers comprising the chalcogen elements, sulfur, selenium, and tellurium. Typically, other elements are added to stabilize the glass. Arsenic sulfide, As2S3, and arsenic selenide As2Se3, germanium arsenic sulfide, germanium arsenic sulfide telluride, and germanium arsenic selenide are examples of chalcogenide glass. Chalcogenide fibers typically do not transmit well in the visible range. The use of high peak power pumps for supercontinuum generation in these fibers risks damage from two-photon absorption, so pumps in the wavelength range greater than 1.5 μm are typically used.
Many of the microchip lasers previously described are capable of laser emissions above 1.5 um, however sources with emissions below 1.5 um and for those applications where power is preferred to remain within a specified wavelength band, a nonlinear element can be used in conjunction with the laser to shift the color of the laser to a longer wavelength. Examples of nonlinear elements are bulk nonlinear material with sufficient transmission at the pump wavelength. Examples are nonlinear crystals such as lithium triborate (LBO), beta barium borate (BBO), zinc germanium phosphide (ZGP), potassium dihydrogen phosphate (KDP), silver thiogallate (AGS), silver selenogallate (AGSe), gallium selenide (GaSe), lithium indium sulfide (LiInS2), lithium indium selenide (LISe), and others. Alternatively, high-efficient conversion is also possible with quasi-phase matched material such as periodically poled lithium niobate, periodically poled potassium titanyl phosphate, or periodically patterned gallium arsenide, and others. Besides conversion in devices with high second order nonlinearity (χ(2)), conversion can be induced through Raman shifting as in the case of Raman converters. Examples of Raman converters can be in the form of a gas-cell, an optical fiber or crystal.
For those alternatives for nonlinear conversion where second order nonlinearity is used for the wavelength conversion, the present invention does not require the use of a cavity, ensuring a compact and stable laser source. The method focuses on wavelength conversion through optical parametric generation, not requiring a set of mirrors to form a cavity. A single pass configuration for the optical parametric generation is preferred. Alternatively, a method where wavelength conversion prior to coupling into the fiber occurs through optical parametric amplification. Optical parametric amplification requires the use of seed laser increasing the complexity of the system, however it can be used to narrow the converted bandwidth, improve the mode or increase the power conversion.
Many applications exist for bright broadband infrared sources beyond about 2 μm. Of particular interest are light sources in the chemical and biological “fingerprint region” from 3-12 μm for biological and chemical sensing and sources within the atmospheric transmission windows from 2-5 μm and 8-12 μm for infrared countermeasures and certain radar (LIDAR) applications. Other applications for such sources include infrared illuminators and infrared sources for hardware-in-the-loop testing. Supercontinuum sources in the infrared would enable these applications. For these applications, the size and weight of the light source are of particular importance. In particular there is growing interest in portable sources (weight on the order of 20 kg, dimensions on the order of 20 cm×20 cm×20 cm). Current inventions do not address the size and weight limitations currently needed.
Shaw (U.S. Pat. No. 7,133,590) teaches a method of generating supercontinuum in a chalcogenide fiber, either conventional core/clad fiber or microstructured photonic crystal fiber within the range of 2 to 14 μm by launching pump light into a chalcogenide fiber whereby the input pump light is broadened by several nonlinear mechanisms in said fibers. However, the invention describes supercontinuum generation in fibers wherein the pump light propagates at a wavelength that is in the anomalous or near-zero dispersion.
Islam (U.S. Pat. No. 7,519,253) teaches a system and method to generate said broadband supercontinuum in either chalcogenide, fluoride, or tellurite fiber with a pump light consisting of a short pulse laser diode with wavelength of shorter than 2.5 μm and pulse width of at least 100 ps with one or more optical amplifier chains and a nonlinear fiber with anomalous dispersion at the diode wavelength that modulates the diode though modulation instability. In addition to the modulation instability stage, the invention requires the use of an amplification stage after the laser pump, increasing the weight and complexity of any device based on this invention. The invention does not teach how to overcome the challenges with the use of other pumps systems such as microchip lasers, and those whose wavelength lie in the normal dispersion regime of the fiber. It also requires the amplification of the pump signal to at least 500 W peak power in a second element such as a fiber amplifier.
Shaw (U.S. Pat. No. 7,809,030) teaches a method for converting light to the infrared through the use of AsS chalcogenide fibers pumped by an optical parametric oscillator. The invention does not disclose how to overcome the challenges with the use of other chalcogenide fiber materials such as AsSe, and GaAsSeTe, as well as the challenges associated with the use of nonlinear conversion without the use of a cavity.
Shaw (U.S. patent application Ser. No. 13/742,563) teaches a method for generating supercontinuum light in the mid-infrared through the use of a fiber based pump source. Although the invention does focus on propagation in the normal dispersion regime, it does not address the challenges involved in using a micro-optic packaged system or bulk system for pumping a fiber within this regime.
Zayhowski (“Miniature sources of subnanosecond 1.4-4.3 μm pulses with high peak power,” in Advanced Solid State Lasers 34, TuA11 (2000), “Miniature gain-switched lasers,” in Advanced Solid-State Lasers 50, WA1 (2001)) teach a method for fabricating compact miniature laser sources with high peak power and narrow bandwidths centered on wavelengths within the range of 1 to 4.3 μm. The cited work does not teach a method for generating broadband wavelength emission within this range or how to extend the range further into the infrared.